Reverse osmosis and nanofiltration membranes are used to separate dissolved or dispersed materials from feed streams. The separation process typically involves bringing an aqueous feed solution into contact with one surface of the membrane under pressure so as to effect permeation of the aqueous phase through the membrane while permeation of the dissolved or dispersed materials is prevented.
Both reverse osmosis and nanofiltration membranes typically include a thin film discriminating layer fixed to a porous support, collectively referred to as a “composite membrane”. Ultrafiltration and microfiltration membranes may also have a composite arrangement. The support provides physical strength but offers little resistance to flow due to its porosity. On the other hand, the discriminating layer is less porous and provides the primary means of separation of dissolved or dispersed materials. Therefore, it is generally the discriminating layer which determines a given membrane's “rejection rate”—the percentage of the particular dissolved material (i.e., solute) rejected, and “flux”—the flow rate per unit area at which the solvent passes through the membrane.
Reverse osmosis membranes and nanofiltration membranes vary from each other with respect to their degree of permeability to different ions and organic compounds. Reverse osmosis membranes are relatively impermeable to virtually all ions, including sodium and chlorine ions. Therefore, reverse osmosis membranes are widely used for the desalination of brackish water or seawater to provide relatively non-salty water for industrial, commercial, or domestic use because the rejection rate of sodium and chlorine ions for reverse osmosis membranes is usually from about 95 to about 100 percent.
Nanofiltration membranes are usually more specific for the rejection of ions. Generally, nanofiltration membranes reject divalent ions, including radium, magnesium, calcium, sulfate, and carbonate. In addition, nanofiltration membranes are generally impermeable to organic compounds having molecular weights above about 200 Daltons. Additionally, nanofiltration membranes generally have higher fluxes at comparable pressures than reverse osmosis membranes. These characteristics render nanofiltration membranes useful in such diverse applications as the “softening” of water and the removal of pesticides from water. As an example, nanofiltration membranes generally have a sodium chloride rejection rate of from about 0 to about 95 percent but have a relatively high rejection rate for salts such as magnesium sulfate and in some cases organic compounds such as atrazine.
Among particularly useful membranes for reverse osmosis and nanofiltration applications are those in which the discriminating layer is a polyamide. The polyamide discriminating layer for reverse osmosis membranes is often obtained by an interfacial polycondensation reaction between a polyfunctional amine monomer and a polyfunctional acyl halide monomer (also referred to as, polyfuntional acid halide) as described in, for example, U.S. Pat. No. 4,277,344, which is incorporated herein by reference. The polyamide discriminating layer for nanofiltration membranes is typically obtained via an interfacial polymerization between a piperazine or an amine substituted piperidine or cyclohexane and a polyfunctional acyl halide as described in U.S. Pat. Nos. 4,769,148 and 4,859,384, both incorporated in their entirety by reference. Another way of obtaining polyamide discriminating layers suitable for nanofiltration is via the methods described in, for example, U.S. Pat. Nos. 4,765,897; 4,812,270; and 4,824,574. These patents describe changing a reverse osmosis membrane, such as those of U.S. Pat. No. 4,277,344, into a nanofiltration membrane.
Composite, polyamide membranes are typically prepared by coating a porous support with a polyfunctional amine monomer, most commonly coated from an aqueous solution. Although water is a preferred solvent, non-aqueous solvents may be utilized, such as acetyl nitrile and dimethylformamide (DMF). A polyfunctional acyl halide monomer (also referred to as acid halide) is subsequently coated on the support, typically from an organic solution. Although no specific order of addition is necessarily required, the amine solution is typically coated first on the porous support followed by the acyl halide solution. Although one or both of the polyfunctional amine and acyl halide may be applied to the porous support from a solution, they may alternatively be applied by other means such as by vapor deposition, or neat.
Means for improving the performance of membranes by the addition of constituents to the amine and/or acyl halide solutions are described in the literature. For example, U.S. Pat. No. 4,950,404, issued to Chau, describes a method for increasing flux of a composite membrane by adding a polar aprotic solvent and an optional acid acceptor to the aqueous amine solution prior to interfacially polymerizing the amine with an polycarboxylic acid halide. Similarly, U.S. Pat. Nos. 6,024,873; 5,989,426; 5,843,351; 5,733,602; 5,614,099; and 5,576,057 to Hirose, et al. describes the addition of selected alcohols, ethers, ketones, esters, halogenated hydrocarbons, nitrogen-containing compounds and sulfur-containing compounds having a solubility parameter of 8 to 14 (cal/cm3)1/2 to the aqueous amine solution and/or organic acid halide solution prior to interfacial polymerization.
Methods of improving membrane performance by post-treatment are also known. For example, U.S. Pat. No. 5,876,602 to Jons, et al. describes treating a polyamide composite membrane with an aqueous chlorinating agent to improve flux, lower salt passage, and/or increase membrane stability to base. U.S. Pat. No. 5,755,964 to Mickols discloses a process wherein the polyamide discriminating layer is treated with ammonia or selected amines, e.g., butylamine, cyclohexylamine, and 1,6 hexane diamine. U.S. Pat. No. 4,765,897 to Cadotte discloses the post treatment of a membrane with a strong mineral acid followed by treatment with a rejection enhancing agent. U.S. Pat. Nos. 4,765,897; 5,876,602 and 5,755,964 are incorporated herein by reference.
Membranes having higher flux at standard operating pressures, or which are capable of maintaining flux at relatively lower operating pressures are desired. Moreover, membranes having higher rejection rates while achieving improved flux and/or lower pressure requirements are also desired. Methods for making such membranes, particularly those readily adaptable to commercial scale membrane fabrication are also desired.